1. Field of the Invention
The present invention relates to a fan.
2. Description of the Related Art
Performance of electronic devices has been improved more and more, and therefore the amount of heat generated inside a casing of the electronic devices has been increasing rapidly. Fans have been used in order to reduce the temperature rise in electronic components of the electronic devices.
Fans are used mainly in the following two ways.    (a) To discharge hot air inside a casing of electronic devices to the outside.    (b) To supply an air flow directly to an electronic component which generates heats, thereby reducing the temperature rise of it.
In case of (a), fans are required to provide a large air flow amount and a high static pressure. In case of (b), fans have to provide a desired flow rate distribution in addition to the characteristics required in the case (a). Please note that the flow rate distribution is a distribution of a flow rate of an air flow discharged from an air outlet of a fan. Quietness is also important in both cases. Moreover, fans must have the strength designed in accordance with their operating environment, for example.
In usual fans, an air flow discharged from an air outlet tends to spread outwardly in a radial direction of an impeller because of a centrifugal force generated by rotation of the impeller. In case of (b), however, it is necessary to supply the air flow to a heat-generating electronic component without allowing the air flow to spread outwardly, because a cooling efficiency becomes higher as the air flow amount delivered to the heat-generating electronic component increases.
In order to prevent spreading of the air flow, some fans are provided with stationary vanes at their air outlets. In other fans, ribs are provided between an outer casing and a motor supporting portion such that an axial height of the ribs decreases as they move outwardly in a radial direction of an impeller (see Japanese Patent Unexamined Publication No. 2006-17117, for example).
When the stationary vanes are provided, a noise is caused by interference of an air flow from the impeller with the stationary vanes. That is, measures against the interference noise are required. More specifically, the flow amount of the air flow from the impeller tends to increase from inner ends to outer ends of rotating vanes of the impeller in the radial direction of the impeller. Thus, the interference noise also tends to increase outwardly in the radial direction. The measures against the interference noise should be taken considering the above. In addition, the strength of the stationary vanes should be also taken into consideration.